The invention relates to a method for modulating a signal, the method using continuous phase modulation and comprising encoding and frequency modulation of a signal to be transmitted.
A modulation method used on a transmission path is a significant parameter when new data transmission systems are developed. Because of losses occurring on the transmission path and because of transmission path capacity, data symbols to be transmitted cannot be transmitted over the transmission path as such, but the symbols must be modulated using a suitable method so as to obtain good transmission path capacity and transmission quality.
The bandwidth required by transmission is a significant factor particularly in radio systems. The aim is to achieve maximum transmission capacity by using a narrow bandwidth. On the other hand, the aim is to provide a transmitter and a receiver as easily and advantageously as possible. In radio systems, the aim is generally to use a modulation method having a constant envelope, because a C-class amplifier solution can then be used. The C-class amplifiers are simple in structure and advantageous in efficiency. This is particularly relevant as far as terminal power consumption is concerned.
There are several prior art modulation methods having a constant envelope, including Minimum Shift Keying MSK, Gaussian Minimum Shift Keying GMSK, Tamed Frequency Modulation TFM and Continuous Phase Modulation CPM. The GMSK method is used in the GSM cellular radio system. It has a narrow frequency spectrum and high performance, whereas data transmission rates are not very high. The coded CPM methods usually have a narrow frequency spectrum and high performance, making high data rates possible. However, equipments required become complex in structure, for which reason these methods have not been used in prior art systems.
An object of the invention is to provide a method and arrangement implementing the method, enabling high data rate transmission in a narrow frequency band without complex equipments required. This is achieved by the method of the type described in the introduction, which is characterized in that the encoding performed is differential encoding and that a signal to be transmitted before the frequency modulation is multiplied by a factor of the form xcfx80/(2m), where m is a positive integer greater than one.
The invention also relates to an arrangement for modulating a signal to be transmitted, the arrangement comprising an encoder and a frequency modulator. The arrangement of the invention is characterized in that the encoder is a differential encoder and that before the frequency modulator, the arrangement comprises means for multiplying the signal to be transmitted by a factor of the form xcfx80/(2m), where m is a positive integer greater than one.
The preferred embodiments of the invention are disclosed in the dependent claims.
The method and arrangement of the invention provide many advantages. The number of feasible states on a unit circle can be minimized by using the selected factor, thus providing a receiver of reduced complexity. On the other hand, bit error ratio obtained by the method of the invention, indicating the ability of the system to tolerate errors, is higher than in GMSK, but if the transmission band remains unchanged, the data rates obtained are substantially higher. Consequently, the method of the invention enables more data to be transmitted using the same frequency band. As compared with the coded CPM methods, the equipments needed, the transceiver in particular, are substantially easier to implement.